Many unicellular and multicellular organisms have been made containing genetic material which is not otherwise normally found in the cell or organism. For example, bacteria, such as E. coli, have been transformed with plasmids which encode heterologous polypeptides, i.e., polypeptides not normally associated with that bacterium. Such transformed cells are routinely used to express the heterologous gene to obtain the heterologous polypeptide. Yeasts, filamentous fungi and animal cells have also been transformed with genes encoding heterologous polypeptides. In the case of bacteria, heterologous genes are readily maintained by way of an extra chromosomal element such as a plasmid. More complex cells and organisms such as filamentous fungi, yeast and mammalian cells typically maintain the heterologous DNA by way of integration of the foreign DNA into the genome of the cell or organism. In the case of mammalian cells and most multicellular organisms such integration is most frequently random within the genome.
Transgenic animals containing heterologous genes have also been made. For example, U.S. Pat. No. 4,736,866 discloses transgenic non-human mammals containing activated oncogenes. Other reports for producing transgenic animals include PTC Publication No. WO82/04443 (rabbit .beta.-globin gene DNA fragment injected into the pronucleus of a mouse zygote); EPO Publication No. 0 264 166 (Hepatitis B surface antigen and tPA genes under control of the whey acid protein promotor for mammary tissue specific expression); EPO Publication No. 0 247 494 (transgenic mice containing heterologous genes encoding various forms of insulin); PTC Publication No. W088/00239 (tissue specific expression of a transgene encoding factor IX under control of a whey protein promotor); PTC Publication No. W088/01648 (transgenic mammal having mammary secretory cells incorporating a recombinant expression system comprising a mammary lactogen-inducible regulatory region and a structural region encoding a heterologous protein); and EPO Publication No. 0 279 582 (tissue specific expression of chloramphenicol acetyltrans-ferase under control of rat S-casein promotor in transgenic mice). The methods and DNA constructs ("transgenes") used in making these transgenic animals also result in the random integration of all or part of the transgene into the genome of the organism. Typically, such integration occurs in an early embryonic stage of development which results in a mosaic transgenic animal. Subsequent generations can be obtained, however, wherein the randomly inserted transgene is contained in all of the somatic cells of the transgenic animals.
Transgenic plants have also been produced. For example, U.S. Pat. No. 4,801,540 to Hiatt, et al., discloses the transformation of plant cells with a plant expression vector containing tomato polygalacturonase (PG) oriented in the opposite orientation for expression. The anti-sense RNA expressed from this gene is capable of hybridizing with endogenous PG mRNA to suppress translation. This inhibits production of PG and as a consequence the hydrolysis of pectin by PG in the tomato.
While the integration of heterologous DNA into cells and organisms is potentially useful to produce transformed cells and organisms which are capable of expressing desired genes and/or polypeptides, many problems are associated with such systems. A major problem resides in the random pattern of integration of the heterologous gene into the genome of cells derived from multicellular organisms such as mammalian cells. This often results in a wide variation in the level of expression of such heterologous genes among different transformed cells. Further, random integration of heterologous DNA into the genome may disrupt endogenous genes which are necessary for the maturation, differentiation and/or viability of the cells or organism. In the case of transgenic animals, gross abnormalities are often caused by random integration of the transgene and gross rearrangements of the transgene and/or endogenous DNA often occur at the insertion site. For example, a common problem associated with transgenes designed for tissue-specific expression involves the "leakage" of expression of the transgenes. Thus, transgenes designed for the expression and secretion of a heterologous polypeptide in mammary secretory cells may also be expressed in brain tissue thereby producing adverse effects in the transgenic animal. While the reasons for transgene "leakage"and gross rearrangements of heterologous and endogenous DNA are not known with certainty, random integration is a potential cause of expression leakage.
One approach to overcome problems associated with random integration involves the use gene of targeting. This method involves the selection for homologous recombination events between DNA sequences residing in the genome of a cell or organism and newly introduced DNA sequences. This provides means for systematically altering the genome of the cell or organism.
For example, Hinnen, J. B., et al. (1978) Proc. Natl. Acad. Sci. U.S.A., 75, 1929-1933 report homologous recombination between a leu2.sup.+ plasmid and a leu2.sup.- gene in the yeast genome. Successful homologous transformants were positively selected by growth on media deficient in leucine.
For mammalian systems, several laboratories have reported the insertion of exogenous DNA sequences into specific sites within the mammalian genome by way of homologous recombination. For example, Smithies, O., et al. (1985) Nature, 37, 230-234 report the insertion of a linearized plasmid into the genome of cultured mammalian cells near the .beta.-globin gene by homologous recombination. The modified locus so obtained contained inserted vector sequences containing a neomycin resistance gene and a sup F gene encoding an amber suppressor t-RNA positioned between the .delta. and .beta.-globin structural genes. The homologous insertion of this vector also resulted in the duplication of some Of the DNA sequence between the .delta. and .beta.-globin genes and part of the .beta.-globin gene itself. Successful transformants were selected using a neomycin related antibiotic. Since most transformation events randomly inserted this plasmid, insertion of this plasmid by homologous recombination did not confer a selectable, cellular phenotype for homologous recombination mediated transformation. A laborious screening test for identifying predicted targeting events using plasmid rescue of the sup marker in a phage library prepared from pools of transfected colonies was used. Sib selection utilizing this assay identified the transformed cells in which homologous recombination had occurred.
A significant problem encountered in detecting and isolating cells, such as mammalian and plant cells, wherein homologous recombination events have occurred lies in the greater propensity for such cells to mediate non-homologous recombination. See Roth, D. B., et al. (1985) Proc. Natl. Acad. Sci. U.S.A., 82 3355-3359; Roth, D. B., et al. (1985), Mol. Cell. Biol., 2599-2607; and Paszkowski, J., et al. (1988), EMBO J., 7, 4021-4026. In order to identify homologous recombination events among the vast pool of random insertions generated by non-homologous recombination, early gene targeting experiments in mammalian cells were designed using cell lines carrying a mutated form of either a neomycin resistance (neo.sup.r) gene or a herpes simplex virus thymidine kinase (HSV-tk) gene, integrated randomly into the host genome. Such exogenous defective genes were then specifically repaired by homologous recombination with newly introduced exogenous DNA carrying the same gene bearing a different mutation. Productive gene targeting events were identified by selection for cells with the wild type phenotype, either by resistance to the drug G418 (neo.sup.r) or ability to grow in HAT medium (tk.sup.+). See, e.g., Folger, K. R., et al. (1984), Cold Spring Harbor Symp. Quant. Biol., 49, 123-138; Lin, F. L. et al. (1984), Cold Spring Harbor Symp. Quant. Biol., 49, 139-149; Smithies, O., et al. (1984), Cold Spring Harbor Symp. Quant. Biol., 49, 161-170; Smith, A. J. H., et al. (1984), Cold Spring Harbor Symp. Quant. Biol., 49, 171-181; Thomas K. R., et al. (1986), Cell, 41, 419-428; Thomas, K. R., et al. (1986), Nature, 324, 34-38; Doetschman, T., et al. (1987), Nature, 330, 576-578; and Song, Kuy-Young, et al. (1987), Proc. Natl. Acad. Sci. U.S.A., 84, 6820-6824. A similar approach has been used in plant cells where partially deleted neomycin resistance genes reportedly were randomly inserted into the genome of tobacco plants. Transformation with vectors containing the deleted sequences conferred resistance to neomycin in those plant cells wherein homologous recombination occurred. Paszkowski, J., et al. (1988), EMBO J., 1, 4021-4026.
A specific requirement and significant limitation to this approach is the necessity that the targeted gene confer a positive selection characteristic in those cells wherein homologous recombination has occurred. In each of the above cases, a defective exogenous positive selection marker was inserted into the genome. Such a requirement severely limits the utility of such systems to the detection of homologous recombination events involving inserted selectable genes.
In a related approach, Thomas, K. R., et al. (1987), Cell, 51, 503-512, report the disruption of a selectable endogenous mouse gene by homologous recombination. In this approach, a vector was constructed containing a neomycin resistance gene inserted into sequences encoding an exon of the mouse hypoxanthine phosphoribosyl transferase (Hprt) gene. This endogenous gene was selected for two reasons. First, the Hprt gene lies on the X-chromosome. Since embryonic stem cells (ES cells) derived from male embryos are hemizygous for Hprt, only a single copy of the Hprt gene need be inactivated by homologous recombination to produce a selectable phenotype. Second, selection procedures are available for isolating Hprt.sup.- mutants. Cells wherein homologous recombination events occurred could thereafter be positively selected by detecting cells resistant to neomycin (neo.sup.R) and 6-thioguanine (Hprt.sup.-).
A major limitation in the above methods has been the requirement that the target sequence in the genome, either endogenous or exogenous, confer a selection characteristic to the cells in which homologous recombination has occurred (i.e. neo.sup.R, tk.sup.+ or Hprt.sup.-). Further, for those gene sequences which confer a selectable phenotype upon homologous recombination (e.g. the Hprt gene), the formation of such a selectable phenotype requires the disruption of the endogenous gene.
The foregoing approaches to gene targeting are clearly not applicable to many emerging technologies. See, e.g. Friedman, T. (1989), Science, 244, 1275-1281 (human gene therapy); Gasser, C. S., et al., Id., 1293-1299 (genetic engineering of plants); Pursel, I. G., et al., Id. 1281-1288 (genetic engineering of livestock); and Timberlake, W. E., et al., Id. et al., 13--13, 1312 (genetic engineering of filamentous fungi). Such techniques are generally not useful to isolate transformants wherein non-selectable endogenous genes are disrupted or modified by homologous recombination. The above methods are also of little or no use for gene therapy because of the difficulty in selecting cells wherein the genetic defect has been corrected by way of homologous recombination.
Recently, several laboratories have reported the expression of an expression-defective exogenous selection marker after homologous integration into the genome of mammalian cells. Sedivy, J. M., et al. (1989), Proc. Nat. Acad. Sci. U.S.A., 86, 227-231, report targeted disruption of the hemizygous polyomavirus middle-T antigen with a neomycin resistance gene lacking an initiation codon. Successful transformants were selected for resistance to G418 . Jasin, M., et al. (1988), Genes and Development, 2, 1353-1363 report integration of an expression-defective gpt gene lacking the enhancer in its SV40 early promotor into the SV40 early region of a gene already integrated into the mammalian genome. Upon homologous recombination, the defective gpt gene acts as a selectable marker.
Assays for detecting homologous recombination have also recently been reported by several laboratories. Kim, H. S., et al. (1988), Nucl. Acid. S. Res., 16, 8887-8903, report the use of the polymerase chain reaction (PCR) to identify the disruption of the mouse hprt gene. A similar strategy has been used by others to identify the disruption of the Hox 1.1 gene in mouse ES cells (Zimmmer, A. P., et al. (1989), Nature, 338, 150-153) and the disruption of the En-2 gene by homologous recombination in embryonic stem cells. (Joyner, A. L., et al. (1989), Nature, 338, 153-156).
It is an object herein to provide methods whereby any predetermined region of the genome of a cell or organism may be modified and wherein such modified cells can be selected and enriched.
It is a further object of the invention to provide novel vectors used in practicing the above methods of the invention.
Still further, an object of the invention is to provide transformed cells which have been modified by the methods and vectors of the invention to contain desired mutations in specific regions of the genome of the cell.
Further, it is an object herein to provide non-human transgenic organisms, which contain cells having predetermined genomic modifications.
The references discussed above are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention.